Wetting current diagnostics

ABSTRACT

A circuit for diagnostic testing includes a current source coupled to a power source and configured to provide wetting current along a path to a load control switch, a current sensor connected in series with the current source along the path, the current sensor being configured to generate a current sensor signal indicative of a current level along the path, a voltage measurement unit having an input terminal coupled to a node along the path through which the wetting current flows to reach the load control switch, the voltage measurement unit being configured to detect a state of the load control switch based on a voltage at the node, and a controller coupled to the current sensor and the voltage measurement unit, the controller being configured to determine a wetting current diagnostic condition in accordance with the current level and the detected state.

RELATED APPLICATION

This patent application is a divisional of co-pending, U.S. patentapplication Ser. No. 14/624,778, filed on Feb. 18, 2015.

FIELD OF INVENTION

The present embodiments relate to sensed switching.

BACKGROUND

Sensed switches are often used to control the operation of loads insteadof powered switches. Powered switches are disposed serially with a loadto directly control current delivered to the load. In contrast, sensedswitches control the load current indirectly. The state of the switch isinstead sensed with a low current signal. The opportunity to use a lowcurrent voltage measurement leads to reduced wiring harness complexity,weight, and costs. In complex electrical systems with numerousswitch-controlled loads, such as automobile vehicles, the cost savingsmay be considerable.

Determining the state of a sensed switch typically involves a voltagecomparison. For example, a voltage level dictated by the state of theswitch is compared with a threshold voltage. The voltage level isideally not dependent on the voltage drop across the switch contacts.But unfortunately, the switch contacts oxidize over time due to humidityand contamination, increasing the resistance presented by the switchitself. The increased resistance results in an increased sensed voltage,thereby increasing the risk of incorrect operation. Switch contactoxidation may be especially challenging in connection with normally openswitches, i.e., switches with contacts that close upon application of anexternal force.

The oxidation challenge presented by sensed switches is not applicableto the powered switch approach. In powered switches, the current levelsare high enough to burn off any oxidation of the switch contacts.Because the current levels may be much lower with sensed switches, awetting current is used to remove the oxidation from the switchcontacts. The wetting current is typically a temporary current level ofthe current that flows through the switch contacts when the switchtransitions from open to closed. The temporary current level issufficient to remove the oxidation. A circuit used to detect the stateof the switch may also be configured to control the application of thewetting current.

Unfortunately, over time, faults may develop along the current path tothe switch. For example, a fault may arise in the wiring harness betweenthe control circuit and the switch. Some faults may inhibit the deliveryof the wetting current to the switch.

BRIEF DESCRIPTION OF THE DRAWINGS

The components and the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the variousembodiments. Moreover, in the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a schematic circuit diagram of an exemplary circuit configuredto provide wetting current diagnostics for an external load controlswitch in accordance with one embodiment.

FIGS. 2-4 are schematic circuit diagrams of exemplary current sensor andmeasurement units of the circuit of FIG. 1 in accordance with variousembodiments.

FIG. 5 is a schematic circuit diagram of an exemplary voltagemeasurement unit of the circuit of FIG. 1 in accordance with oneembodiment.

FIG. 6 is a graphical representation of exemplary current level rangesassociated with a number of valid and fault operating conditions inaccordance with one embodiment.

FIG. 7 is a process flow diagram of an exemplary method of wettingcurrent diagnostics testing in accordance with one embodiment.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Embodiments of methods, circuits, and other devices or units for wettingcurrent diagnostic testing are described. The diagnostic testing may bedirected to determining or confirming that a wetting current for a loadcontrol switch flows when the load control switch is closed. The methodmay be implemented by a circuit or other unit or device directed tosensing the state of the switch. The wetting current diagnostic testingcircuitry may be integrated with circuitry configured to detect thestate of the load control switch and/or provide the wetting current. Thedevices, units, or circuits described herein may thus provide thecapability for built-in self testing.

The diagnostic testing may be correlated with the state of the loadcontrol switch (e.g., open or closed). The correlation may involvedetecting the state of the load control switch and/or the condition whenwetting current is flowing. The diagnostic testing may be configuredsuch that the wetting current may be detected when (e.g., only when) theload control switch is closed. In these and other ways, the testing maydistinguish between wetting current and other current flows. Additionalaspects of the diagnostic testing may involve other examples ofcoordinated voltage and current measurements. Through such coordination,diagnostic testing may be implemented when the load control switch isopen. Such testing may include, for example, diagnosing the operation ofa voltage measurement unit and/or diagnosing a leakage current faultcondition and/or one or more other open switch fault conditions.

The disclosed embodiments may have a circuit topology that sensescurrent along the current path to the load control switch. A variety ofdifferent current sensors may be used. The sensed current may bemeasured against multiple current thresholds.

The diagnostic testing may be directed to detecting, and differentiatingbetween, several different fault conditions. Exemplary fault conditionsinclude high and low wetting current faults, a switch detection fault, aleakage fault, and a system (e.g., voltage) offset fault. The faultconditions may involve both current and voltage measurements.Additional, alternative or fewer fault conditions may be tested. Forexample, in some cases, the presence of a leak is not tested.

FIG. 1 depicts an electrical system 10 in which a load control switch 12is provided to control the operation of a load 14. The load 14 may be amotor, lamp, or any other type of load. The load 14 may be configuredfor direct current (DC) or alternating current (AC) operation. In thisembodiment, the load 14 is powered by a DC power source V+ that alsoprovides power for the load control switch 12. In other cases, differentvoltage sources are used. For example, the power source for the load 14may be a high voltage AC power source, and the power source for the loadcontrol switch 12 may be a low voltage DC power source, which may or maynot be derived from or otherwise related to the high voltage AC powersource.

In some cases, the electrical system 10 is a vehicular electricalsystem. The DC power source V+ may be a 12 Volt vehicular battery. Inthese and other cases, the load 14 is one of a number of loadscontrolled by respective load control switches 12. The nature andcharacteristics of the electrical system 10 may vary considerably.

The load control switch 12 is a sensed switch. The state of the loadcontrol switch 12 determines whether power is delivered to the load 14.As a sensed switch, the load control switch 12 is not disposed in thecurrent path of the power delivered to the load 14. In some cases, theload control switch 12 is a push-button switch or other normally openload control switch. For example, in vehicular embodiments, the loadcontrol switch 12 may be a push-button switch, such as a power windowpush-button switch, directly actuated by an operator or other occupantof the vehicle. The load control switch 12 may be actuated in a varietyof other ways. For example, the load control switch 12 may be actuatedthrough the opening of a vehicle door or other indirect actuationmechanism.

In the embodiment of FIG. 1, the load control switch 12 is configured asa switch to ground. As a switch to ground, the closure of the loadcontrol switch 12 establishes a connection to ground. The connection toground lowers a voltage level, which is sensed to control the deliveryof power to the load 14. In other embodiments, the load control switch12 is configured as a switch to battery or other voltage source. Theload control switch 12 may be configured to establish a connection toany reference voltage.

A control module, unit, or circuit 16 senses the connection to ground(or other voltage change) at an evaluation node 17 of the controlcircuit 16. The load control switch 12 is coupled to the node 17. Duringoperation, a wetting current I_(wet) for the load control switch 12flows through the node 17 when the load control switch 12 is closed. Inthe switch to ground example of FIG. 1, the connection to ground lowersthe voltage level at the node 17. The control unit 16 is configured todetect the lowered voltage level at the node 17 and, thus, the state ofthe load control switch 12.

The control circuit 16 controls the delivery of power to the load 14 inaccordance with the state of the load control switch 12. The controlcircuit 16 may thus be referred to or configured as a switch detectionunit. In the switch to ground example of FIG. 1, the control circuit 16allows power to reach the load 14 upon detecting the lowered voltagelevel at the node 17. To this end, the control circuit 16 includes apower transistor 18 that acts as a switch to allow current to flowthrough the load 14.

Activation of the power transistor 18 is controlled by a logiccontroller 20 (or other controller) and a detection circuit 22 of thecontrol circuit 16. The logic controller 20 activates and deactivatesthe power transistor during a non-diagnostic mode of operation inresponse to a control signal provided by the detection circuit 22. Inthe example of FIG. 1, the logic controller 20 also controls operationof the detection circuit 22 and other components of the control circuit16 during a diagnostic mode of operation. For example, the functioningof the control circuit 16 may be dictated by the state of the loadcontrol switch 12 in both non-diagnostic and diagnostic modes ofoperation.

The power transistor 18 is disposed in the current path of the load 14rather than the load control switch 12. In this example, the powertransistor 18 is a discrete power field effect transistor (FET) device.In other examples, the power FET device is part of an integratedcircuit. Other types of transistor devices may be used, such as bipolarjunction transistor devices. Other types of switches may be used,including relays.

The control circuit 16 may be one of several circuits in the electricalsystem 10. Multiple loads 14 may be controlled by each control circuit16. Some of the components of the control circuit 16 may be replicated,with a respective instance of the component being provided for each load14. For example, the control circuit 16 may include multiple powertransistors 18, one for each load 14. Alternatively, a single controlcircuit 16 may control multiple loads 14.

In the example of FIG. 1, the logic controller 20, the detection circuit22, and other components of the control circuit 16 are disposed on asingle integrated circuit (IC) chip 28. In other cases, multiple ICchips may be used. For example, the logic controller 20 and thedetection circuit 22 may be disposed on respective IC chips. The logiccontroller 20, the detection circuit 22, the FET device 18, and anyother components of the control unit 16 may be mounted on a commoncircuit board, and/or disposed in a common housing, and/or otherwiseintegrated in any other manner or to any other desired extent. The loadcontrol switch 12 and the load 14 may be external to the board orhousing of the control unit 16, or be otherwise disposed remotely fromthe control unit 16. Wiring 24, 26 may be used to establish connectionsbetween the control unit 16 and the load control switch 12 and the load14, respectively. For example, a wiring harness may be used to carry thewiring 24, 26 from a door, dashboard, or other panel or portion of avehicular interior, to another location in the vehicle at which thecontrol unit 16 is located, such as an electronics cabinet under thedashboard. The manner in which the load control switch 12 and the load14 are connected to the control unit 16 may vary. For example,components in addition to the wiring harness may be used, including, forinstance, fuses.

The length of the wiring 24 may be sufficiently extensive to presentsignificant parasitic capacitance and resistance. In some cases, one ormore external capacitors and/or one or more external resistors aredisposed in series with the wiring 24. For example, a series resistormay be included for purposes of electrostatic discharge (ESD)protection.

The wiring 24 couples the load control switch 12 to a pin 32 of thecontrol circuit 16. The pin 32 may be one of a set of pins of the ICchip in which the control circuit 16 is integrated. The packaging of thecontrol circuit 16 and the components thereof may vary. The nature ofthe pin 32 may thus be configured as a post, solder bump, or otherconnection point of the packaging of the detection circuit 22. In theembodiment of FIG. 1, the pin 32 corresponds with the node 17. The pin32 and the node 17 may thus be disposed at the same voltage level. Inother cases, the node 17 and the pin 32 may not constitute a commonnode. For example, a series resistor may be disposed between the node 17and the pin 32.

The logic controller 20 may be configured to implement a number of logicfunctions. The functions include controlling the power transistor 18. Tothat end, a control signal may be generated by the control circuit 20and provided to a gate or other control terminal of the power transistor18. The functions also include analysis of the state of the load controlswitch 12. The state of the load control switch 12 is used to determinewhether to generate or change the control signal to the power transistor18. The logic controller 20 and the detection circuit 22 are alsoresponsive to the state of the load control switch 12 to provide thewetting current.

The logic controller 20 performs functions directed to wetting currentdiagnostics. The logic controller 20 may implement one or morediagnostic routines of the wetting current function as described below.Each function or routine may be implemented by a separate logic block,software or firmware module, or other component of the logic controller20. The logic blocks or other components of the logic controller 20directed to implementing these functions may be integrated to anydesired extent. For example, a single routine may be implemented by thelogic controller 20 to provide all of the functions.

The control circuit 16 includes a current source 34 coupled to a powersource V+. The current source 34 is configured to provide the wettingcurrent along a path to the load control switch 12. The current source34 may be a variable current source to provide different levels ofcurrent to the load control switch 12, including, for instance, wettingand sustaining current levels. Any number of current levels may besupported. The current source 34 may be or include an active circuit,such as current amplifier or other active current source circuit. Inother cases, the current source 34 may be or include a passive circuit,such as a resistor or resistor network.

The control circuit 16 includes a current sensor 36 connected in serieswith the current source 34 along the path to the load control switch 12.The current sensor 36 generates a signal (e.g., a voltage signal)indicative of the current flowing along the path to the load controlswitch 12. In this example, the current sensor 36 is disposed betweenthe current source 34 and the power source V+. In other cases, thecurrent sensor 36 is disposed between the current source 34 and the pin32. The relative positions of the current source 34 and the currentsensor 36 along the path may thus vary from the example shown.

The current sensor 36 may include an active circuit, examples of whichare provided in connection with FIGS. 2-4. In the example of FIG. 1, thelogic controller 20 provides a current control signal to the currentsource 34 to establish the current level. Alternatively or additionally,the current sensor 36 may be or include a passive circuit, such as aresistor. The voltage generated across the resistor is indicative of thecurrent flowing along the path to the load control switch 12. In somecases, the same resistor(s) may serve as both the current source 34 andthe current sensor 36.

In the embodiment of FIG. 1, the control circuit 16 also includes acurrent measurement unit 38 coupled to the current sensor 36 to receivethe current sensor signal. The current measurement unit 38 compares thecurrent level indicated by the current sensor signal with one or morethreshold current levels. The current measurement unit 38 is configuredto generate one or more output signals indicative of the comparison(s).The output signal(s) may be provided to the logic controller 20. Thecurrent measurement unit 38 may be integrated with the current sensor 36and/or the logic controller 20 to any desired extent. For example, thelogic controller 20 may include a comparator, analog-to-digitalconvertor, and/or other circuitry to perform the comparison(s). Examplesof current measurement units 38 are shown and described in connectionwith FIGS. 2-4.

The detection circuit 22 includes a voltage measurement unit 40 todetect the closure of the load control switch 12. The voltagemeasurement unit 40 has an input terminal coupled to the node 17 (e.g.,the pin 32). The voltage measurement unit 40 is configured to detect astate of the load control switch 12 based on a voltage at the node 17.An output signal indicative of the state of the load control switch 12(and/or the measured voltage) is provided by the voltage measurementunit 40 to the logic controller 20.

The voltage at the node 17 may be compared to a reference voltage level.In some cases, a source of the reference voltage is coupled to anotherinput terminal of the voltage measurement unit 40. In such cases, thevoltage measurement unit may include a comparator, as shown in theexample of FIG. 5. Alternatively, the reference voltage may be comparedto an internal or inherent reference voltage level. For example, thevoltage measurement unit 40 may include an analog-to-digital converter.

The logic controller 20 is coupled to the current sensor 36 and thecurrent measurement unit 38 to receive a signal indicative of thecurrent level along the wetting current path. The logic controller 20 isconfigured to determine a wetting current diagnostic condition inaccordance with the current level. The wetting current diagnosticcondition may be determined relative to one or more threshold currentlevels. For example, a desired range for the wetting current may bespecified via two threshold current levels. The logic controller 20 maythen provide a low current fault alert if the current level falls belowthe current range, and provide a high current fault alert if the currentlevel is above the range. Alternatively, the current range may bespecified by a single threshold current level along with an acceptableamount of deviation above and below the threshold current level, such asplus or minus 1 milliamp.

The logic controller 20 is also coupled to the voltage measurement unit40 to provide the wetting current diagnostics in accordance with thedetected state. The wetting current diagnostic condition may thus bebased on both the output signal from the voltage measurement unit 40 aswell as the output generated by the current sensor 36. For example, thelogic controller 20 may be configured to implement one or morediagnostic routines (a first set) when the voltage measurement unit 40detects that the state of the load control switch 12 is open, and thenone or more diagnostic routines (a second set) when the state of theload control switch 12 is closed. As described below, the first set maybe configured to confirm a low current level with an open switch, and todetermine whether the state of the load control switch is detectedcorrectly. The second set may be configured to compare the current levelwith the threshold current level(s).

The correlation between the voltage and current measurements may bebased on the following assumptions, or desired or expected operationalcharacteristics. Little to no current is expected when the load controlswitch 12 is open (i.e., open switch testing in the first set). Forexample, any current level below about 50 microAmperes for a 12 Voltautomotive system may be considered acceptable. A desired level ofwetting current is expected when the load control switch 12 is closed(i.e., closed switch testing in the second set). The diagnostic routinesmay be configured to implement the open switch testing first, e.g., as apreliminary matter to confirm that the switch state is being detectedproperly.

FIG. 6 is a chart 600 depicting one example of how the logic controller20 correlates operation of the voltage measurement unit 40 and thecurrent measurement unit 38. Details regarding the open switch testingin the first set is shown in a column 602. Details regarding the closedswitch testing in the second set is shown in a column 604. Various faultconditions may be identified in each set based on the measured currentlevel.

The diagnostic routine(s) in the open switch testing may proceed asfollows. While receiving an indication of an open state of the loadcontrol switch 12, the logic controller 20 may provide a switchdetection fault if the current level falls between a current thresholdI₁ and a current threshold I₂. Measured current levels between thecurrent thresholds I₁ and I₂ are within a range of an expected wettingcurrent level I_(wet) (i.e., the current level expected to be reachedwith the load control switch 12 closed). The logic controller 20 may beconfigured to interpret current levels within that range as indicativeof the voltage measurement unit 40 and/or some related component of thecontrol circuit 16 falsely indicating switch closure. Alternatively oradditionally, the current levels in that range may be considered theresult of an internal short. In the example of FIG. 6, however, thelogic controller 20 provides an internal short fault if the currentlevel exceeds the current threshold I₂.

The diagnostic routine(s) in the open switch testing may also attempt toidentify one or more additional fault conditions. In the example of FIG.6, the logic controller 20 tests for a leakage current fault conditionin which current levels fall between threshold current levels I₂ and I₃correspond with the leakage current fault condition. The thresholdcurrent level I₃ corresponds with the maximum current allowed with theload control switch 12 open. Alternatively, a leakage current fault isfound when the measured current exceeds the threshold current level I₃.The threshold current level I₃ may be greater than a valid (or desired)open switch current level I_(open) to account for noise or otherfactors. The leakage current fault may be indicative of leakage currentoccurring at some point downstream of the current sensor 36.

The diagnostic routines in the closed switch testing may also involveone or more threshold current levels. The threshold current levels maycorrespond with the threshold current levels used in connection with theopen switch testing. In other cases, one or more different thresholdcurrent levels are used. In the example of FIG. 6, the threshold currentlevels I₁ and I₂ are used to distinguish between valid closed switchcurrent and two different fault conditions. A low wetting current faultis generated or provided when measured currents fall below the thresholdcurrent level I₂. A high wetting current fault is generated or providedwhen measured currents are above the threshold current level I₁. Thethreshold current levels I₁ and I₂ may thus define a range of acceptablewetting current levels. The threshold current levels may be spaced fromthe desired or optimal closed switch current level I_(wet) to accountfor noise or other factors. In other embodiments, the range ofacceptable current levels is established by the closed switch currentlevel I_(wet) and an acceptable offset amount.

The control circuit 16 and the logic controller 20 may also beconfigured to test the voltage measurements apart from the currentmeasurements. In the example of FIG. 1, the control circuit 16 includesa voltage reference source 42 and a switch 44 that selectively couplesthe voltage reference source 42 to the voltage measurement unit 40. Thelogic controller 20 may be configured to select a state of the switch44. In the example of FIG. 1, the switch 44 is controlled with a switchcontrol signal generated by the logic controller 20. During normaloperation, the switch 44 is positioned to couple an input terminal ofthe voltage measurement unit 40 to the pin 32 and, thus, the node 17.During testing, the switch 44 is positioned to couple the input terminalof the voltage measurement unit 40 to the voltage reference source 42.

The voltage measurement testing may include one or more voltagereference levels. For example, the voltage reference source 42 may beconfigured to generate multiple reference voltages (e.g., 3 and 6Volts). The logic controller 20 may select one of the reference voltagesor otherwise direct the operation of the voltage reference source 42 viaa reference control signal, as shown in FIG. 1. The reference voltagesmay be independently generated from the power source so that any faultsin the other components of the control circuit 16 are not introducedinto the voltage measurement testing. The reference control signal mayspecify a reference voltage level from a predetermined set of discretereference voltage levels or from a continuous range of reference voltagelevels.

The logic controller 20 and the detection circuit 22 may be integratedto any desired extent. For example, a comparator and/or other circuitryof the detection circuit 22 may be integrated with the logic controller20. In some cases, a microcontroller, such as a mixed signal FPGA, mayinclude both an analog-to-digital converter to act as the voltagemeasurement unit 40 and/or one or more logic blocks to implement thelogic functionality of the logic controller 20.

Additional functions may also be provided by the logic controller 20.For example, the control circuit 20 may be configured to select one ofmultiple levels of the current to be provided to the load control switch12. The wetting current is provided to the load control switch 12 toburn off the oxidation on the contacts of the load control switch 12.After the wetting current is applied for a period of time, the logiccontroller 20 may be configured to lower the current level for, e.g.,power savings. The lower current level may be associated with, orconfigured as, a sustaining current, i.e., a current level sufficient tomaintain the closed state of the load control switch 12. Multiplesustaining current levels (or other current levels) may be used. Otheraspects of the wetting current may be controlled, including, forinstance, the duration of application of the wetting current. In othercases, the duration and/or other aspects of the wetting current aredetermined passively. The term wetting current is used herein to referto either the current and/or voltage level sufficient to remove theoxidation, but the actual level, duration, and/or other characteristicsof the wetting current may vary.

The logic controller 20 may be or include a microcontroller or othercontroller, or a general microprocessor or an application-specificmicroprocessor, such as an application-specific integrated circuit(ASIC). In other embodiments, a field-programmable gate array (FPGA) orother controller may be used as the logic controller 20. The logiccontroller 20 may include one or more processors and one or morememories in which instructions to configure the processor are stored.The instructions are executed by the processor of the logic controller20 to implement the logic used during the non-diagnostic and diagnosticmodes of operation. The logic controller 20 may include any combinationof firmware and general-purpose memory to store instructions to beexecuted during operation.

The configuration of the current source 34 may vary. For example, thecurrent source 34 may be or include various types of pull-up circuitryto support the operation of the load control switch 12. The pull-upcircuitry may include a resistor or other circuit between the voltagesource V+ and the node 17. In some examples, the pull-up circuitry maybe configured as or include a current source or current regulating loop.The resistor or other circuit thus pulls up the voltage level at thenode 17 to or toward the power source when the load control switch 12 isopen. The pull-up circuitry may include additional circuitry (e.g., oneor more active circuits, including one or more transistor devices) toestablish multiple levels of current provided via the node 17 to theload control switch 12. The multiple current levels may include awetting current level (e.g., 15-20 milliamps), and one or more lowercurrent levels (e.g., 1-2 milliamps) sufficient to sustain the closureof the switch. The load control switch 12 may be closed, and a wettingcurrent may be applied for a predetermined time period (e.g., 20milliseconds). After the predetermined time period, a sustaining current(or sealing current or fret current) may be applied. Changing from thewetting current to the sustaining current lowers power dissipation,which may be useful in conserving charge stored in the battery. In theexample of FIG. 1, the pull-up circuitry is responsive to a currentcontrol signal generated by the logic controller 20. The current controlsignal may be provided to lower the current that flows through the loadcontrol switch 12 from the level of the wetting current to the lowerlevel of the sustaining current after operation for a predeterminedamount of time at the wetting current level.

FIGS. 2-5 depict various exemplary configurations of the current sensor36, the current measurement unit 38, and the voltage measurement unit40. Other configurations may be used.

For example, the disclosed embodiments are not limited to the specificsensor-measurement unit pairings shown in FIGS. 2-4. Other pairings maybe used, such that a current sensor shown in one figure may be pairedwith a measurement unit shown in another figure.

FIG. 2 depicts a circuit 50 configured to provide current sensing andcurrent measurement unit. The circuit 50 includes a current sensor 52and a current measurement unit 54 to generate an output signalindicative of the current flowing through a current source 56. In thisexample, the sensing element of the current sensor 52 is a diode 58. Thediode 58 is disposed in the path along which the wetting current flowsto reach the load control switch 12 (FIG. 1). A bipolar transistor 60 ofthe current measurement unit 54 is connected as a current mirror acrossthe diode 58 in a current mirror arrangement. In this example, thecurrent mirror drives a resistive load 62. A voltage across theresistive load 62 is developed at the collector of the bipolartransistor 60. The current measurement unit 54 further includes avoltage comparator 64 (e.g., an operational amplifier) to compare thevoltage across the resistive load 62 with a reference voltage, VT. Inthis example, the collector is connected to the inverting terminal of anoperational amplifier of the voltage comparator 64 to measure thevoltage (e.g., through comparison with a reference voltage provided atthe non-inverting terminal). The output of the voltage comparator 64 isthus indicative of the current level in the path relative to a referencecurrent level represented by the reference voltage.

FIG. 3 depicts a circuit 70 that includes a resistive current sensor 72and an inverter-based measurement unit 74. In this example, the currentsensor 72 includes a single resistor but other resistor arrangements maybe used. The measurement unit 74 includes a field effect transistor(FET) device 76, a digital inverter 78, and a current source 80 as theload. In operation, the current source 56 in the wetting current pathcauses a voltage drop across the resistive current sensor 72. Thevoltage drop modifies the extent to which the FET device 76 isactivated. If activated enough, the FET device 76 pulls up the voltageat the input terminal of the digital inverter 78 to the power source.The input terminal voltage is then inverted in accordance with thebuilt-in threshold of the digital inverter 78 to produce the outputsignal. The configuration of the current source 80 may vary, as avariety of active loads may be used.

FIG. 4 depicts a circuit 90 configured to provide current sensing andmeasurement via a common base amplifier arrangement. In this example,the sensing element is provided by a resistor 92 disposed in the wettingcurrent path with the current source 56. As the current through theresistor 92 increases, the voltage at the emitter of a bipolartransistor 94 drops. Eventually, the bipolar transistor 94 activates,which activates another bipolar transistor 96 having a base terminal incommon with the bipolar transistor 94. With a current sensor 98 as anactive load, the extent to which the bipolar transistor 96 is activatedchanges the voltage at the inverting input terminal of an operationalamplifier 100. The operational amplifier 98 then compares the voltagewith a reference voltage, VT, to generate an output signal indicative ofthe wetting current level. The configuration of the current source 98may vary, as a variety of active loads may be used.

FIG. 5 depicts an exemplary voltage measurement circuit 110 having acomparator 112 and reference voltage circuitry 114. A reference voltage(or threshold) is generated by the reference voltage circuitry 114 andprovided to one of the input terminals of the comparator 112. A voltageto be measured, such as the voltage at the node 17 (FIG. 1) or a testvoltage, is provided to the other input terminal of the comparator 112.In one example, the comparator 112 is or includes an operationalamplifier. In such cases, either one of the input terminals may beconfigured as an inverting input terminal, while the other is configuredas a non-inverting input terminal. During operation, the voltage to bemeasured is compared to the level of the threshold voltage. The logiccontroller 20 (FIG. 1) is communicatively coupled to an output terminalof the comparator 112 to determine whether to activate the powertransistor 18 (FIG. 1) and provide other control signaling (e.g.,wetting current diagnostics-related signaling).

The reference voltage may be generated in various ways. The referencevoltage circuitry 114 may be coupled to the power source V+to establishthe reference voltage. In some cases, the reference voltage circuitry114 is or includes a voltage divider arrangement. Other types of circuitarrangements may be used. For example, the threshold circuitry mayinclude one or more active devices.

Other types of voltage measurement circuits may be used. For example,the voltage measurement unit may be or include various types ofanalog-to-digital converters. In analog-to-digital converter examples,the voltage measurement unit may not include reference voltagecircuitry, insofar as the reference voltage is built-in or inherent tothe analog-to-digital conversion.

FIG. 7 shows an exemplary method for wetting current diagnostic testing.The method may be implemented by the control circuits and/or controllersdescribed above. In some cases, another processor or controller may beused to implement the method either in conjunction with theabove-described controllers or separately therefrom. The method includesa sequence of acts or steps, only the salient of which are depicted forconvenience in illustration. Additional, fewer, or alternative acts maybe included. For example, the test sequence may include only testingacts related to current measurement. The ordering of the acts may varyin other embodiments. For example, current measurement data may beobtained before or during the detection of the state of the load controlswitch. As another example, the test sequence may include conductingclosed switch testing before open switch testing, if, for instance, aload control switch is closed at the point at which the method isinitiated.

The method is described in connection with a switch to ground loadcontrol switch arrangement (e.g., as in the embodiment illustrated inFIG. 1). In the following description, the “node” is an evaluation node,such as the node 17 of FIG. 1. The method may alternatively be appliedin connection with a switch to battery arrangement.

The method may be applied to test the wetting current for a normallyopen load control switch. With the load control switch normally open,oxidation of the switch contacts may occur over time. The contacts ofthe load control switch may thus benefit from the application of thewetting current each time that the load control switch is closed. Themethod is directed to testing whether the wetting current is actuallyprovided to the load control switch. Additional aspects of the operationof the wetting current circuitry may also be tested, as describedherein.

The method may begin with, or include, an act 700 directed to testing avoltage measurement unit of the control circuit. In the example of FIG.7, the act 700 includes selecting one or more test input voltages in anact 702, comparing the test input voltage(s) with a voltage reference inan act 704, and generating or otherwise providing a switch detectionfault alert in an act 706 if the output of the comparison is incorrect.The test input voltage(s) may be generated by an independent referencevoltage source. The selection of the test input voltage(s) may involvecontrolling an input selection switch as described above. The voltagemeasurement unit testing may not occur in other embodiments, or may, forinstance, be implemented only occasionally (e.g., upon system startup)relative to the other diagnostic testing.

In act 708, a state of the load control switch is detected. The statedetection may include selecting a measurement switch setting in an act710 if, for instance, the operation of the voltage measurement unit waspreviously tested against one or more test input voltages. The statedetection may also include measuring a voltage at the node through whichthe wetting current passes to the load control switch in an act 712.Such voltage measurement may involve a comparison with a referencevoltage (e.g., using a comparator) or a circuit involving an inherentcomparison (e.g., using an analog-to-digital converter).

Current measurement data may be obtained in an act 714. The act 714 mayoccur before, during, or after detection of the state of the loadcontrol switch. The act 714 may include measuring a current level of thewetting current using a current sensor and a current measurement unit,as described above.

When the detected state is an open state, control is redirected by adecision block 716 to an act 718, in which the controller attempts todetermine whether the voltage measurement unit is operating correctly.The controller may thus determine whether the state of the load controlswitch is detected correctly. The determination may be based on themeasured current level. In this example, the act 718 may includecomparing in act 720 the measured current level to one or more currentthresholds. The current thresholds may correspond with the currentthresholds described in connection with FIG. 6. In this case, a decisionblock 722 determines whether the measured current is within a desiredrange, such as below the current threshold I₃ (FIG. 6). If the measuredcurrent is within the desired range (i.e., below the current thresholdI₃), then the procedure may end. Otherwise, control passes to an act724, in which a fault alert is provided. The fault condition may dependon the measured current level, as described above. For instance, if themeasured current is below the current threshold I₁, then an internalshort fault alert may be provided. If the measured current falls betweenthe current thresholds I₁ and I₂, then a switch detection fault alertmay be provided. If the measured current falls between the currentthresholds I2 and I3, then a leakage current fault alert may beprovided. Additional, alternative, or fewer fault conditions may beidentified. For instance, in some cases, the method may only provide asingle leakage current fault alert if the measured current is outside ofthe desired range.

In other embodiments, some or all of the open switch diagnostic testingis not implemented. For instance, the open switch diagnostic testing maybe conducted only periodically or occasionally to save power.

The closed switch diagnostic testing may begin with an act 726, in whichthe measured current is compared with one or more closed switch currentthresholds. A decision block 728 may then determine whether the measuredcurrent is outside of an acceptable range of current levels. Theacceptable range may be defined by upper and lower threshold currentlevels, such as the levels I₁ and I₂ (FIG. 6). In the example of FIG. 7,if the measured current falls within the acceptable range, then thewetting current circuitry is operating properly, and the method mayrestart or terminate. Otherwise, a wetting current fault notification isprovided in an act 730 when the measured current level falls outside ofthe range of current levels relative to the closed switch currentthreshold(s). A high current fault alert may be provided if the measuredcurrent exceeds the upper closed switch current threshold. A low currentfault alert may be provided if the measured current is below the lowerclosed switch current threshold.

In a first aspect, a circuit for diagnostic testing regarding wettingcurrent provided to a load control switch includes a current sourcecoupled to a power source and configured to provide the wetting currentalong a path to the load control switch, a current sensor connected inseries with the current source along the path, the current sensor beingconfigured to generate a current sensor signal indicative of a currentlevel along the path, a voltage measurement unit having an inputterminal coupled to a node along the path through which the wettingcurrent flows to reach the load control switch, the voltage measurementunit being configured to detect a state of the load control switch basedon a voltage at the node, and a controller coupled to the current sensorand the voltage measurement unit, the controller being configured todetermine a wetting current diagnostic condition in accordance with thecurrent level and the detected state.

In a second aspect, a circuit for diagnostic testing regarding wettingcurrent provided to a load control switch includes a current sourcecoupled to a power source and configured to provide the wetting currentalong a path to the load control switch, a current sensor connected inseries with the current source along the path, the current sensor beingconfigured to generate a current sensor signal indicative of a currentlevel along the path, a current measurement unit coupled to the currentsensor to receive the current sensor signal, the current measurementunit being configured to implement comparisons of the current level withan open switch threshold and a closed switch threshold, a voltagemeasurement unit having an input terminal coupled to a node along thepath through which the wetting current flows to reach the load controlswitch, the voltage measurement unit being configured to detect a stateof the load control switch based on a voltage at the node, and acontroller connected to the current measurement unit and the voltagemeasurement unit, the controller being configured to determine a wettingcurrent diagnostic condition in accordance with the comparisons and thedetected state.

In a third aspect, a method of providing wetting current diagnostics fora load control switch includes detecting a state of the load controlswitch, and measuring a current level of a wetting current. When thedetected state is an open state, a determination is made, based on themeasured current level, whether the state of the load control switch isdetected correctly. When the detected state is a closed state, themeasured current level is compared to a closed switch current threshold.

Although described in connection with load control switches for use invehicles, the disclosed embodiments are not limited to any particulartype or application of load control switches. The load control switchesmay be used to control any type of load. The load control switches arethus not limited to motors (AC or DC motors), lamps, or other types ofloads commonly present on vehicles. The load control switches are thusalso not limited to uses involving 12-Volt batteries or other batteries.

The disclosed embodiments are also compatible with a variety ofdifferent load control switch environments. The wetting currentdiagnostics may be provided regardless of the external resistance and/orcapacitance presented by the wiring harness and/or other components oraspects of the system in which the load control switch is disposed. Thedisclosed embodiments may utilize a voltage threshold established forthe comparator of the detection unit to avoid any requirements forcustomization to a specific switch environment.

Although described in connection with single-pole, single-throwswitches, the disclosed embodiments are not limited to any particulartype of switch. The number of poles may vary. The number of connectionoptions may also vary. For example, the disclosed embodiments may beconfigured for use with double-throw or triple-throw switches.

The wetting current diagnostic testing disclosed embodiments may beuseful in a wide variety of automotive and industrial switchingapplications. The diagnostic testing is well-suited for multi-channelsystems, such as those presented by automotive applications.

While the wetting current diagnostics are useful for normally openswitches, the disclosed embodiments may be used in connection withnormally closed switches and/or other types of switches. The extent towhich wetting current is useful for the load control switch may vary.

The present invention is defined by the following claims and theirequivalents, and nothing in this section should be taken as a limitationon those claims. Further aspects and advantages of the invention arediscussed above in conjunction with the preferred embodiments and may belater claimed independently or in combination.

While the invention has been described above by reference to variousembodiments, it should be understood that many changes and modificationsmay be made without departing from the scope of the invention. It istherefore intended that the foregoing detailed description be regardedas illustrative rather than limiting, and that it be understood that itis the following claims, including all equivalents, that are intended todefine the spirit and scope of this invention.

1. A method of providing wetting current diagnostics for a load controlswitch, the method comprising: detecting a state of the load controlswitch; measuring a current level of a wetting current; when thedetected state is an open state, determining, based on the measuredcurrent level, whether the state of the load control switch is detectedcorrectly; and when the detected state is a closed state, comparing themeasured current level to a closed switch current threshold.
 2. Themethod of claim 1, further comprising providing a wetting current faultnotification when the measured current level falls outside of a range ofcurrent levels relative to the closed switch current threshold.
 3. Themethod of claim 1, wherein determining whether the state of the loadcontrol switch is detected correctly comprises comparing the measuredcurrent level to first and second open switch current thresholds.
 4. Themethod of claim 3, wherein: detecting the state comprises measuring avoltage at a node through which the wetting current passes to the loadcontrol switch; and the method further comprises providing a voltageoffset fault alert if the measured current level falls between the firstand second open switch current thresholds and if the measured voltage isabove a voltage reference.
 5. The method of claim 3, further comprisingproviding a leakage fault alert if the measured current level fallsbetween the first and second open switch current thresholds and themeasured voltage is below the voltage reference.
 6. The method of claim1, further comprising: selecting a test input voltage; and testingoperation of a voltage measurement unit by comparing the test inputvoltage with a voltage reference.
 7. A method of providing wettingcurrent diagnostics for a load control switch, the method comprising:providing, by a current source coupled to a power source, a wettingcurrent along a path to the load control switch; generating, by acurrent sensor connected in series with the current source along thepath, a current sensor signal indicative of a current level along thepath; detecting, by a voltage measurement unit having an input terminalcoupled to a node along the path through which the wetting current flowsto reach the load control switch, a state of the load control switchbased on a voltage at the node; and determining, by a controller coupledto the current sensor and the voltage measurement unit, a wettingcurrent diagnostic condition in accordance with the current level andthe detected state of the load control switch.
 8. The method of claim 7,further comprising: receiving, by a current measurement unit coupled tothe current sensor and to the controller, the current sensor signal;generating, by the current measurement unit, an output signal indicativeof a comparison of the current level with a threshold current level; anddetermining, by the controller, the wetting current diagnostic conditionbased on the output signal.
 9. The method of claim 8, furthercomprising: correlating, by the controller, operation of the voltagemeasurement unit and the current measurement unit.
 10. The method ofclaim 8, further comprising: implementing, by the controller, first andsecond diagnostic routines when the voltage measurement unit detectsthat the state of the load control switch is open and closed,respectively, wherein the first diagnostic routine is configured todetermine whether the voltage measurement unit is operating correctly,and wherein the second diagnostic routine is configured to compare thecurrent level with the threshold current level.
 11. The method of claim7, further comprising: providing a low current alert, by the controller,if the current level falls below a current range; and providing a highcurrent alert, by the controller, if the current level is above therange.
 12. The method of claim 7, further comprising: providing a switchdetection fault alert, by the controller, while receiving an indicationof an open state of the load control switch, if the current level iswithin a range of current levels expected to be reached with the loadcontrol switch closed.
 13. The method of claim 7, further comprising: ifthe current level is above an open threshold current level with the loadcontrol switch in an open position, deactivating, by the controller, thecurrent source to determine a voltage level at the node with the currentsource deactivated; generating, by the controller, a voltage offsetfault alert if the voltage level remains above a threshold voltage levelupon deactivation; and generating, by the controller, a leakage faultalert if the voltage level falls below the threshold voltage level upondeactivation.
 14. The method of claim 7, further comprising: selecting,by the controller, a state of a switch that couples the input terminalof the voltage measurement unit to either a voltage reference source orthe node.
 15. A method of providing wetting current diagnostics for aload control switch, the method comprising: providing, by a currentsource coupled to a power source, a wetting current along a path to theload control switch; generating, by a current sensor connected in serieswith the current source along the path, a current sensor signalindicative of a current level along the path; receiving, by a currentmeasurement unit coupled to the current sensor, the current sensorsignal; comparing, by the current measurement unit, the current levelwith an open switch threshold and a closed switch threshold; detecting,by a voltage measurement unit having an input terminal coupled to a nodealong the path through which the wetting current flows to reach the loadcontrol switch, a state of the load control switch based on a voltage atthe node; and determining, by a controller connected to the currentmeasurement unit and the voltage measurement unit, a wetting currentdiagnostic condition in accordance with the comparisons and the detectedstate.
 16. The method of claim 15, wherein: implementing, by thecontroller, first and second diagnostic routines when the voltagemeasurement unit detects that the state of the load control switch isopen and closed, respectively, wherein the first diagnostic routine isconfigured to detect whether the state of the load control switch isdetected correctly based on the comparisons of the current level withthe open and closed switch thresholds, and wherein the second diagnosticroutine is configured to compare the current level with the closedswitch threshold.
 17. The method of claim 15, wherein the current sensorcomprises a diode disposed in the path and a current mirror connectedacross the diode, and the current measurement unit comprises a voltagecomparator and a resistive load driven by the current mirror, and themethod further comprises: comparing, by the voltage comparator, avoltage across the resistive load with a reference voltage.
 18. Themethod of claim 15, wherein the current sensor comprises a resistordisposed in the path and a transistor coupled to the resistor such thata voltage drop across the resistor controls an extent to which thetransistor is activated.
 19. The method of claim 15, wherein the currentsensor comprises a common base amplifier circuit.
 20. The method ofclaim 15, wherein the current measurement unit comprises an active load.